Abstract:

The partial nucleotide sequences of 18S ribosomal RNA gene were amplified and determined for M.anguillicaudatus and M.albus
 by PCR with four pairs of specific primers we designed. The 18S rRNA sequences of the two species are identical in length, and its length is 1 204 bp (They were deposited in the GenBank database under the  accession Nos.EU120032 and EU120033, respectively). The GC content of 18S rRNA gene of M.anguillicaudatus was 53.74%, and the GC skew of that gene of M.albus was 0.082. Only four nucleotide substitutions were observed, accounting for 9967% identity between the two 18S rRNA sequences, whereas several changes were found in 18S rRNA sequences by  comparison with homologues of  S.salar (982% identity). The sequences of 18S rRNA gene were compared with those of four mitochondrial genes (12S rRNA, 16S rRNA, Cyt b and Dloop) in M.anguillicaudatus, M.albus and S.salar. We found that 18S rRNA gene was more conserved than other four genes within three species, and the evolutionary rate of the Dloop gene was the highest in five genes. The same DNA sequences of 4 other fishes and 11 chordate species were retrieved from GenBank, respectively. The phylogenetic trees (including three trees in fact) for 6 sequences of fishes were derived using 18S rRNA gene and three different analytical approaches: neighbourjoining (NJ), maximum parsimony (MP), and maximum likelihood (ML), and B.lanceolatum （AY428817）was used for the outgroup species. Another phylogenetic trees (including NJ, MP, and ML trees) for 13 animals belonging to Chordata were developed using the same gene
 and methods. The phylogenetic analyses results strongly supported a sister group relationship for the cartilaginous fishes and bony fishes, and favored a fishes + tetrapods clade with high bootstap support values （≥92%）. In addition, a kind of potential bias involving GC content of 18S rRNAs was discussed in metazoans.

Key words: Misgurnus anguillicaudatus, Monopterus albus, 18S rRNA gene, sequence analysis